Interactive Tutorial about Diffraction Reciprocal space IV: Oblique lattice 

Reciprocal space
Interactive examples
Goto 
In this example a monoclinic lattice is simulated. The unit cell
lengths are identical, the angel between the base vectors is 75
degrees. Two by two unit cells are simulated, each containing one
atom at (0 0). This miniature crystal is sufficient in size to
illustrate the geometrical relationships. By increasing the size, we
will just decrease the FWHM of the Bragg reflections. The pictures
below show the atoms and the corresponding Fourier transform. Click
on the pictures for a full screen
version.
The reciprocal space geometry (right image) corresponds to that of the real space. The reciprocal base vectors are of equal length, but the angle between the base vectors is 180  75 = 105 degrees. The vertical reciprocal base vector is normal to the horizontal direct space vector, while the second reciprocal base vector (to the right and slightly down) is normal to the other real space vector (up and slightly right). 
© Th. Proffen and R.B. Neder, 2003 